POTS / Landline / Telephone interface for PIC MCU [CLOSED]

benoitstjean · Posted: Sun Dec 13, 2020 4:01 pm

Hi all,

This is kind of a question I should have asked 20 years ago... I realize that mostly everyone has a cell phone these days but many of us still have good ol'pots lines (or rather, a line that rings a standard POTS phone tied to an IP modem). I don't want to fiddle-around with trying stuff while connected to my modem because I tried something today and the lights were all flashing and I had to reset it! Ooops! I don't want to have to pay for equipment I damaged.

Basically, all I am trying to find is a simple circuit that I can somehow interface to a PIC MCU that will notify it when the line is off-hook and on-hook.... like off-hook = 1 and on-hook = 0... or the opposite, I don't care. I just wand to know the state of the line.

At the moment, I have a 3-component circuit that doesn't require external power and when off-hook, an LED turns-on and when on-hook, the LED is off. This is all nice and fine but the problem here is that this is all powered via the RING line which I had to cut and put components on it.

A phone line is referenced to some ground somewhere, if I measure the voltage between the LED (either side) and a power supply GND, then I get between -50 and -52V DC and when it rings then that's AC.

I realize that I may require a DAA circuit but these are somewhat no longer available. I've used Zarlink's MH88437 in the past but these are no longer manufactured of course.

Has anyone ever built such interface and is willing to share?

Many thanks!

Ben

[EDIT] Tried this with a 4N26 (vs a 4N35) and it didn't work... seems pretty close to the same part, just varying specs...

https://www.theremin.us/Circuit_Library/offhookdcindicator.html

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Go to Digikey and type in: Telecom DAA
There are many chips available from Silicon Labs and IXYS.

benoitstjean · Posted: Sun Dec 13, 2020 5:24 pm

ha!

I was on DK website today searching for like, everything else except DAA. I order there every week. I'll check.

Thanks!

Ben

benoitstjean · Posted: Sun Dec 13, 2020 5:28 pm

But I was thinking more of something super simple with a few parts that doesn't need any type of programming. Doesn't matter if I have to stick a 9V battery of a 12V wall plug, I just wanted something simple. Anyhow, I'll check.

It's not a price issue, it's more to have something as simple as possible.

Thanks again.

Ben

temtronic · Posted: Sun Dec 13, 2020 7:18 pm

Here's something like I kinda remember making, sort of, been awhile....
https://www.theremin.us/Circuit_Library/offhookdcindicator.html
I know the cicuit was powered by the phoneline, though I thought I had a zener in the circuit. 4-5 parts, dang reliable and optoisolated..THAT .. was the big feature. Pretty sure it was in parallel, not in series like above circuit.
I did a LOT of 'answering service' interfacing. Still have the 'hot' spare computer here (was $68K back in '86'...). My energy control system used 'Bell wires', solid copper, could signal 15 miles no problem, true one wire.

Crazy thing is I found some 4N37s here today, while looking for a DS18B20.

sigh...
Jay

benoitstjean · Posted: Sun Dec 13, 2020 7:26 pm

Hi Jay,

Thanks, that's the link I have in my initial post in the [EDIT].

I tried it and didn't get it to work. It's unclear in that diagram which side of the [TELEPHONE] is <ring> and which side is <tip> but still, couldn't get anything out on pin 4.... so either that diagram just doesn't work or my 4N26 was dead. I didn't try much after, it was dinner time.

When I go to work some time soon I will check if I have some 4N35's kicking around.

Thanks for your time.

Ben

temtronic · Posted: Sun Dec 13, 2020 7:35 pm

Update.. You need to have a FW bridge between 'tip-ring' and the opto.....
It's common for someone to flip tip->ring, ring->tip, so polarity is reveresed. 'phones' don't care...silly analog stuff.....
Jogs my memory that there were 2 'DIP' pkgs on the perfboard, 4 Rs...
dang, it's kinda scary what I can recall....
Now if you need some BIX connectors.........I have those.

Jay

djsb · Joined: 29 Jan 2011 Posts: 36

This might be helpful

https://docuri.com/download/an-introduction-to-telephone-line-interfacing-using-the-pic-micro-controller_59a8d499f581719e12acff14_pdf

benoitstjean · Posted: Sun Dec 20, 2020 9:40 am

Wow, this is quite the complete document. I think it should probably be renamed to <The most complete guide to everything you've ever wanted to know about how to hook-up a PIC to a landline>.... hahaha!

Thank you so much for sharing. I will definitely look into it.

Have a wonderful year 2021.

Ben